1. Field of the Invention
The present invention relates to a method of generating an area light source by scanning, an area light source and a laser projection television using the same. More particularly, the invention relates a scanning area light source having uniform brightness, non-interference and high efficiency, and a laser projection television using such a scanning area light source.
2. Description of the Related Art
From 90' in the last century, the television display technique is promoted with uninterrupted growth of high and new techniques. Various kinds of television color displays have been developed in the world, such as liquid crystal displays (LCD) technique, rear projection display technique, plasma display technique, laser color display technique, overlapping LED display technique and the like. These display techniques as mentioned above have some advantages but disadvantages, and two techniques being the most related art to the present invention now are available.
The first kind of displays is used in the laser television researched and developed by Laser Display Technique (LDT), Daimler Benz and Schneider Rundfunkwerke S. A. in Germany in joint venture (reported in Laser Focus World, November 1997, P52). As shown in FIG. 9, the Laser television above described comprises a red light laser 1-R, a green light laser 1-G, a blue light laser 1-B, a red light modulator 8-R, a green light modulator 8-G, a blue light modulator 8-B, mirrors 13-R and 13-B, a dichroic sheet for reflecting red light and transmitting green light 14a, a dichroic sheet for reflecting blue and transmitting green light 14b, a horizontal rotating mirror 3, a Galvanometric mirror 5, a projection objective 9 and a projection screen 10. The laser television technique above described should face some problems. It is very difficult to adjust dynamic equilibrum and to perform signal synchronization due to extreme rotating rate of the horizontal rotating mirror 3. And the latter has to keep excellent performance in preventing dust, otherwise the surfaces of the mirror should be damaged by dust. Several light intensity modulators require extreme high responding speed and very wide region in gray level modulation. In addition, such a laser television is high in cost and has problems to be resolved so that it is difficult to be produced in batch.
The second is produced by Hitachi Co. in Japan, as shown in FIG. 10, a projection unit comprises an incandescent lamp 15 as source, a beam expander 16, a dichroic sheet for reflecting red light and transmitting green and blue light 17, a dichroic sheet for reflecting blue light and transmitting green light 18, mirrors 19, 20, 21 and 22, a blue channel 2-D spatial light modulator 23, a green channel 2-D spatial light modulator 24, a red channel 2-D spatial light modulator 25, a combination prism 26, a projection objective 27 and a projection screen 28. However, the second television display technique also has some problems to be resolved: short operating lifetime of the incandescent lamp which generally operates for 2000 hours, high energy loss, narrow color gamut, low color saturation and complex structures caused by the dichroic reflection system.
As the developing of technique and the presence of solid state laser, a new laser color projection television which possesses advantages of two techniques above described is developed to provide high display quality.
Generally, a laser expander is a normal apparatus used to the field of laser application, which is to provide an area light source having a predetermined caliber by expanding a fine laser beam. It usually adopts a collimator tube, namely, a telescope system.
The laser expander using a collimator tube comprises a condenser lens, an aperture and a collimating lens. The laser expander is designed so that a rear focus of the condenser lens is located at the location of a fore focus of the collimating lens, and the aperture is arranged at this location to improve the collimating performance of the projected light. The laser expander is operated as the following: a laser beam generated from a laser device pass through the condenser lens to focus on the rear focus of the condenser, which is the fore focus of the collimating lens. Then the laser beam pass through the aperture located at the focus to irradiate the collimating lens, and further pass through the collimating lens to generate expanded parallel lights. Such a structure can implement the beam expanding of a laser beam, and it is a normal structure used to expand a laser beam. Unfortunately, since the expanded beam is coherent light, in the case of requiring incoherent lights, this structure has limitation and a new expander is needed. For example, in a laser display using a 2-D spatial light modulator, an incoherent area laser source with uniform brightness is requested. Otherwise, some strong interference fringes caused by the laser interference effect would be overlapped on images displayed so as to deteriorate imaging effects. In addition, the cross section of the light beam expanded by this expander is circular, and the brightness of light follows Gaussian distribution instead of uniform distribution. Therefore, this kind of expanded light could not be used as an illumination light source of laser display.
In the existing art, most of light scanning techniques are used to imaging purpose, instead of generating an area light source having uniform brightness. A main characteristic of these techniques exists in that the scanning points are separated from each other, which indicates each of the pixels. For example, the light scanning technique in a laser printer or a laser photocomposer is used for imaging purpose.